1. Field of the Invention
The present invention generally relates to a communication system, and in particular, to a system and method for controlling power of a Mobile Station (MS) in a communication system.
2. Description of the Related Art
Extensive research on next generation communication systems are being conducted to provide users with high-speed services having various Quality of Service (QoS) levels. Meanwhile, power control schemes used in current communication systems can normally be classified as downlink (forward) power control schemes and uplink (reverse) power control schemes according to direction of power control, and can be classified as open-loop power control schemes and closed-loop power control schemes according to whether a transmitter, or a Base Station (BS), receives feedback information from a receiver, or an MS.
In downlink power control schemes, power control is performed in a BS. When the channel condition is good as a distance between the BS and an MS is short, i.e. as the MS is located in a center area of the BS, or as there is no shadowing due to obstacles, the BS allows the MS to transmit signals with minimum possible transmission power, thereby reducing interference to neighboring BSs. When channel conditions are poor, the BS increases power of transmission signals as high as needed within a possible range so the MS may normally receive transmission signals from the BS. In uplink power control schemes, power control is performed in an MS. The MS serves as a transmitter and the BS serves as a receiver, and they control power in the same manner as that in downlink power control schemes.
In open-loop power control schemes, a transmitter (BS or MS) performing power control, independently determines channel quality of a receiver (MS or BS), and performs power control depending on the channel quality, and open-loop power control scheme control power based on reversibility between downlink and uplink channels. Reversibility between downlink and uplink channels, as used herein, means that MSs having the same (or similar) location with respect to a BS will experience similar path attenuation due to similar path attenuation based on a distance from the BS, the similar antenna gain based on antenna patterns, similar shadowing effect by the topology, similar multi-path fading, etc. That is, in open-loop power control schemes, based on reversibility between downlink and uplink channels, a transmitter spontaneously estimates signal reception quality of a receiver, calculates necessary transmission power depending on the estimated signal reception quality, and then transmits signals with the calculated transmission power.
In closed-loop power control schemes, unlike in open-loop power control schemes, a transmitter controls necessary transmission power based on signal reception quality of a receiver from which it has received a feedback channel, without independently determining a channel quality. In such a closed-loop power control scheme, overhead for feedback channels occurs undesirably. However, because a transmitter can acquire information on the channel quality at the receiver, a closed-loop power control scheme can accurately control power of transmission signals, as compared to an open-loop power control scheme.
However, in a typical communication system, as a number of MSs receiving communication services from a BS increases, a closed-loop power control scheme suffers from an increase in an amount of channel quality information that the BS should receive from MSs over a feedback channel. In addition, a closed-loop power control scheme suffers from an increase in overhead of a feedback channel allocated for receiving channel quality information. Further, in a typical communication system, when a BS transmits power control information of MSs to the MSs according to channel quality information fed back from the MSs, an increase in a number of MSs increases an amount of power control information that the BS should transmit to the MSs, and the increase in the amount of power control information for the MSs causes an increase in overhead in the communication system. Therefore, a need exists for a power control scheme for reducing overhead in communication systems.